The aim of this project is to isolate and characterize a protein from adult rat liver that produces a reversible inhibition of the proliferation of liver-derived cells. An improved analytical-scale purification procedure was recently developed that produces a preparation with a specific activity about 1000-fold greater than previously reported. This procedure has been modified for the large-scale purification of the growth inhibitor from 12 x 1000 livers. At the present time a preparation (4 mg total protein) with an ID50 of 1-5 ng/ml has been obtained. The preparation contains at least two separate growth inhibitory polypeptides which appear to have similar activities. Two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) analysis demonstrates that the preparation contains at least 20-30 polypeptides and current efforts are focused on obtaining a pure preparation of the inhibitory activities so that amino acid sequence analysis can be performed. Biological and physicochemical characterization of the liver-derived growth inhibitor(s) has demonstrated that it is different from any known well-characterized growth inhibitory polypeptides including transforming growth factor-beta (TGF-beta), tumor necrosis factor (TNF), interferon (IFN) and mammary derived growth inhibitor (MDGI).